While organic electroluminescent (EL) devices have been known for over two decades, their performance limitations have represented a barrier to many desirable applications. In simplest form, an organic EL device is comprised of an anode for hole injection, a cathode for electron injection, and an organic medium sandwiched between these electrodes to support charge recombination that yields emission of light. These devices are also commonly referred to as organic light-emitting diodes, or OLEDs. Representative of earlier organic EL devices are Gurnee et al, U.S. Pat. No. 3,172,862, issued Mar. 9, 1965; Gurnee, U.S. Pat. No. 3,173,050, issued Mar. 9, 1965; Dresner, “Double Injection Electroluminescence in Anthracene”, RCA Review, 30, 322, (1969); and Dresner U.S. Pat. No. 3,710,167, issued Jan. 9, 1973. The organic layers in these devices, usually composed of a polycyclic aromatic hydrocarbon, were very thick (much greater than 1 μm). Consequently, operating voltages were very high, often greater than 100V.
More recent organic EL devices include an organic EL element consisting of extremely thin layers (e.g. <1.0 μm) between the anode and the cathode. Herein, the term “organic EL element” encompasses the layers between the anode and cathode. Reducing the thickness lowered the resistance of the organic layers and has enabled devices that operate at much lower voltage. In a basic two-layer EL device structure, described first in U.S. Pat. No. 4,356,429, one organic layer of the EL element adjacent to the anode is specifically chosen to transport holes, and therefore is referred to as the hole-transporting layer, and the other organic layer is specifically chosen to transport electrons and is referred to as the electron-transporting layer. Recombination of the injected holes and electrons within the organic EL element results in efficient electroluminescence.
There have also been proposed three-layer organic EL devices that contain an organic light-emitting layer (LEL) between the hole-transporting layer and electron-transporting layer, such as that disclosed by C. Tang et al. (J. Appl. Phys. 65, 3610 (1989)). The light-emitting layer commonly consists of a host material doped with a guest material, otherwise known as a dopant. Still further, there has been proposed in U.S. Pat. No. 4,769,292 a four-layer EL element comprising a hole-injecting layer (HIL), a hole-transporting layer (HTL), a light-emitting layer (LEL) and an electron-transporting/injecting layer (ETL). These structures have resulted in improved device efficiency.
EL devices in recent years have expanded to include not only single color emitting devices, such as red, green and blue, but also white-devices, devices that emit white light. Efficient white light producing OLED devices are highly desirable in the industry and are considered as a low cost alternative for several applications such as paper-thin light sources, backlights in LCD displays, automotive dome lights, and office lighting. White light producing OLED devices should be bright, efficient, and generally have Commission International d'Eclairage (CIE) chromaticity coordinates of about (0.33, 0.33). In any event, in accordance with this disclosure, white light is that light which is perceived by a user as having a white color.
Since the early inventions, further improvements in device materials have resulted in improved performance in attributes such as color, stability, luminance efficiency and manufacturability, e.g., as disclosed in U.S. Pat. Nos. 5,061,569; 5,409,783; 5,554,450; 5,593,788; 5,683,823; 5,908,581; 5,928,802; 6,020,078; and 6,208,077, amongst others.
Notwithstanding all of these developments, there are continuing needs for organic EL device components, such as green light emitting layers, which will provide even lower device drive voltages and hence lower power consumption, while maintaining high luminance efficiencies and long lifetimes combined with high color purity.
U.S. 2005/0058853 discloses OLED devices with green light-emitting layers with a host, a green dopant and a stabilizing dopant where the green dopant has a lower bandgap energy than the stabilizing dopant. Anthracene hosts as well as quinacridone green dopants are described.
U.S. 2008/0102311 discloses OLED devices with light-emitting layers containing bis-9,10-(diarylamino)anthracenes as a green light-emitting compound with anthracene hosts. The light-emitting layer can contain anthracenes, styryls, and quinacridones.
U.S. 2006/0202190; U.S. Pat. Nos. 5,759,444, 6,251,531 and 5,811,834 discloses OLED devices with light-emitting layers containing bis-9,10-(diarylamino)anthracenes as a light-emitting compound. The light-emitting layer can contain anthracenes, styryls, and quinacridones. U.S. Pat. No. 6,743,948 discloses OLED devices with light-emitting layers containing bis-9,10-(diarylamino)anthracenes and bis-9,10-(diarylaminophenyl)anthracenes as light-emitting materials. Anthracene, quinacridone, and stilbene derivatives can also be present
U.S. Pat. No. 6,951,693 discloses OLED devices with light-emitting layers containing bis-9,10-(diarylamino)anthracenes and bis-9,10-(diarylaminophenyl)anthracenes where the aryl substituents of the amine are substituted with styryl groups as light-emitting materials. Anthracene, quinacridone, and stilbene derivatives can also be present.
U.S. Pat. No. 6,929,871 discloses OLED devices with light-emitting layers containing bis-9,10-(diarylamino)anthracenes and bis-9,10-(diarylaminophenyl)anthracenes as light-emitting materials. Other dopants including styrylbenzenes and amine containing dopants can be present.
U.S. 2008/0138655: U.S. 2006/127698; U.S. 2004/0209118; JP2006-253445; EP1722604; and U.S. 2006/033421 discloses OLED devices with light-emitting layers containing bis-9,10-(diarylamino)anthracenes as a green light-emitting compound with anthracene hosts. EP1069628 discloses OLED devices with light-emitting layers containing bis-9,10-(diarylamino)anthracenes as a green light-emitting compound with electron-transporting hosts.
U.S. Pat. Nos. 6,821,644; 6,670,051 and 6,468,675 disclose OLED devices with light-emitting layers containing bis-9,10-(diarylamino)anthracenes and bis-9,10-(diarylaminophenyl)anthracenes where the aryl substituents of the amine are substituted with styryl groups as light-emitting materials.
U.S. 2006/068221 discloses OLED devices with light-emitting layers containing bis-9,10-(diarylamino)anthracenes as a light-emitting compound and polyaromatic compounds which are not 9,10-disubstituted anthracenes as host. Quinacridones and bis-styryls can be present. U.S. Pat. No. 6,534,199 discloses OLED devices with light-emitting layers containing styrylamines as green light-emitting compounds with anthracene hosts.
Quinacridones are described in U.S. Pat. No. 5,593,788; JP 09-13026A and U.S. Pat. No. 6,664,396. Biphenylstyrylamines are described in U.S. 2006/0093856; WO2007086701; JP2007-254386; and Ho et al, Appl. Phys. Lett., 91(8), 083515/1 (2007).
However, these devices do not necessarily have all desired EL characteristics in terms of high luminance, low drive voltages, and sufficient operational stability. Notwithstanding all these developments, there remains a need to improve efficiency and reduce drive voltage of OLED devices, as well as to provide embodiments with other improved features.